A photographing apparatus such as a digital camera or a camcorder includes a lens driving assembly that moves a lens in an optical axis direction. The lens driving assembly may perform an autofocus function of automatically adjusting a focus of a lens on a subject. To this end, the lens driving assembly may include an autofocus driving unit that moves the lens in the optical axis direction with respect to an image sensor.
The autofocus driving unit may include a lead screw to move the lens in the optical axis direction and a driving source that rotates the lead screw. As the driving source, various motors such as a direct current motor, an ultrasound motor, or a step motor may be used.
For example, in consideration of the weight and noise of the driving source, a step motor may be used as a driving source of the autofocus driving unit. Position precision and a driving speed of the autofocus driving unit including the lead screw and the step motor may be adjusted according to a pitch of screw threads of the lead screw.
If the pitch of the screw threads of the lead screw is increased to provide a high driving speed, position precision whereby the lens may be stopped may decrease.
On the contrary, if the pitch of the screw threads is reduced to improve position precision, the number of revolutions of the step motor has to increase, and this may cause noise or vibration.
Meanwhile, when the pitch of screw threads is maintained and a step motor having a relatively high number of steps per revolution are used in order to improve position precision, more magnet polarities are needed in a motor of the same size. Accordingly, an effective magnetic surface area in the motor is reduced, to thus result in a decrease in driving efficiency and driving torque.